Chemical Composition Matters! The Role of Nepetalactone Isomers in Tick Repellency

Repellent and electrophysiological activity of catnip (Nepeta cataria L.) essential oils (EOs) and the main chemical components were evaluated against nymphs and adult female Ixodes scapularis ticks. Horizontal repellency bioassays were conducted on three selected catnip essential oils, and only nepetalactone-rich catnip oil exerted the strongest repellency. Five varieties of catnip essential oils (EOs) were analyzed using gas chromatography-mass spectrometry (GC-MS) to determine their chemical compositions. Among them, nepetalactone isomers (trans-cis isomer 76.6 ± 0.3% and cis-trans isomer 16.9 ± 0.4% relative overall abundance) were detected in only one EO variety. Other major compounds detected in the other EO varieties were α-pinene and menthone. To further assess the role of nepetalactone, isomers were isolated by liquid chromatography, and the cis-trans isomer was further prepared through an epimerization reaction of the trans-cis isomer. The electrophysiological response of adult tick females to a known attractant and host volatile (i.e., butyric acid), pre- and post-exposure to catnip EO and main component nepetalactone isomers, was recorded. Exposure was assessed using a fumigation assay, which revealed that pre-exposure to catnip EO and individual nepetalactone isomers significantly reduced the tick response to butyric acid. Horizontal repellency bioassays were conducted using the two nepetalactones, and it was found that the cis-trans isomer was mainly responsible for the observed activity by repelling 84.0 ± 7.5% of ticks after 2 hours. These results highlight the importance of chemical compositions of complex mixtures (such as EO) and the presence of specific compounds and isomers in eliciting the repellent effect in ticks.